Data communication is the movement of computerencoded information from one point to another by means of a transmission system. Data communication results in nearly instantaneous information exchange over long distances.
Data communication links data terminal equipment (DTE) such as a terminal, printer or computer that transmits or receives data. Data communication equipment (DCE) is a device attached between a DTE and the communication channel that manipulates the transmitted signal or data. The DCE usually comprises a microprocessor and random access memory (RAM). The communication channel is often a telephone network, although it could be a cellular network, a digital communication network, or a satellite network.
The information sent by a transmitter DTE (TXDTE) to a receiver DTE (RXDTE) consists of a sequence of characters. The information generally contains a significant amount of redundancy. The information, therefore, may be compressed so that it can be transmitted in less time over a communication channel.
Among known data compression methods is the Ziv-Lempel '78 algorithm ("ZL78"). In the ZL78 algorithm, the transmit DCE (TXDCE) records the history of recently transmitted data by storing the strings in a vocabulary (also referred to as the "vocabulary tree") stored in the TXDCE RAM. By comparing successive elements of the current data with the vocabulary, redundant data is found. The TXDCE, instead of sending the entire redundant sequence, sends a codeword which points to the location of the earlier occurrence of the redundant data in the vocabulary tree. Data compression occurs whenever the number of bits required to send the codeword is less than the number of bits in the redundant data sequence.
At the other end of the channel, the receiver DCE (RXDCE) maintains a vocabulary in the RXDCE RAM similar to that maintained by the TXDCE. Upon receipt of the codeword from the TXDCE, the RXDCE uses the codeword to find the redundant data sequence in the vocabulary. The RXDCE then transmits the data sequence to the TXDTE.
As the information is transmitted, the TXDCE builds a vocabulary according to a set of rules. The vocabulary is a tree structure data base with various levels of interconnected nodes. A full description of a procedure for building the tree, updating the tree, deleting nodes from the tree and adding nodes to the tree may be found in Clark, U.S. Pat. No. 5,153,591 and Welsh, U.S. Pat. No. 4,558,302. Such a tree structure has been implemented in V.42bis applications for the CCITT (Comite Consultatif International de Telegraphie et Telephonie).
In a typical implementation, each node in the vocabulary tree contains down, left, and right pointers. The down pointer of a node points to a linked list of children nodes The left and right pointers are used to link the node to other children nodes under a common parent node. Searching in the vocabulary tree involves moving down the tree and traversing across the linked list of children nodes. Thus, the search operation involves a sequential search of a linked list, which is expensive in terms of microprocessor usage. Adding and deleting from the vocabulary tree involves updating the left and right pointers of adjacent nodes in the linked list, and may involve updating the down pointer of the parent node. Thus, the add and delete operations are expensive in terms of microprocessor usage.
These methods for encoding and decoding compressed data therefore require significant amounts of microprocessor usage. Thus, there is a need for a method and device using less microprocessor usage.